SAN FRANCISCO -- Facebook's plan to spread its online social network to other websites could be detoured by regulators looking into privacy concerns that have raised the ire of federal lawmakers.

Four senators said Tuesday that Facebook needs to make it easier for its 400 million users to protect their privacy as the site opens more avenues for them to share their interests and other personal information.

The Federal Trade Commission already had been examining the privacy and data collection practices of Facebook and other social networks, the agency confirmed Tuesday.

Then last week, Facebook announced a proposed expansion that irked Sen. Charles Schumer, D-N.Y., and, he said, many Web surfers who called his office to complain.

Having built one of the Web's most popular hangouts, Facebook is trying to extend its reach through new tools called "social plug-ins." These enable Facebook's users to share their interests in such products as clothes, movies and music on other websites. For instance, you might hit a button on Levis.com indicating you like a certain style of jeans and then recommend a movie on another site. That information about the jeans and the movie might be passed along to other people in your Facebook network, depending on your privacy settings.

Facebook says all this will help personalize the Web for people. It stresses that no personal information is being given to the dozens of websites using the new plug-ins.

Still, it means that information that hadn't been previously communicated could get broadcast to your friends and family on Facebook.

And Facebook is indeed sharing some personal information with three websites that Facebook hopes will demonstrate how online services can be more helpful when they know more about their users. The sites with greater access to Facebook's data are business review service Yelp, music service Pandora and Microsoft Corp.'s Docs.com for word processing and spreadsheets.

Facebook users who don't want to be part of the company's expansion have to go through their privacy settings and change their preferences.

Schumer thinks the onus instead should be on Facebook to get users' explicit consent, a process known as "opting in."

"They have sort of assumed all their users want their information to be given far and wide, which is a false assumption," Schumer said.

"We welcome a continued dialogue with you and others because we agree that scrutiny over the handling of personal data is needed as Internet users seek a more social and interactive experience," a Facebook vice president, Elliot Schrage, wrote in a letter to Schumer.

Schumer called Facebook's response inadequate and said his staff planned to meet with the company today.

Meanwhile, the FTC indicated it will weigh into the debate at some point.

"Our plan is to develop a framework that social networks and others will use to guide their data collection, use, and sharing practices," said Jessica Rich, deputy director of the FTC's Bureau of Consumer Protection.

Schumer pledged to introduce legislation that would expand the FTC's powers over Facebook and other Internet social networks if the regulatory agency doesn't feel it has the authority to require more straightforward privacy controls.

The political pressure could undermine Facebook's ambition to create a more social, open Web that could make it easier to aim online advertising at consumers based on their presumed interests. Facebook would probably thrive in a more communal Internet because it has amassed a huge database of personal information since Zuckerberg set up its website in a Harvard dorm room six years ago.

If Facebook's plans pan out, it could change the way people think of social networking. Instead of communicating on a closed website, Facebook's users could interact with one another over the entire Web. More sharing could spawn more customized websites that look different to each person visiting, depending on their friends and preferences.

While Zuckerberg has likened his vision to an online nirvana, critics see another hole in the crumbling walls of online privacy.

Facebook is moving from being a social network about sharing with friends "to a service that is about collecting and sharing information about you with advertisers so they can more closely tailor ads to you," said Ginger McCall, staff counsel at the Washington-based Electronic Privacy Information Center.